This unit relates generally to apparatus and methods for manufacturing MEMS (micro-electromechanical systems) by forming a multiplicity of such devices on a wafer (such as silicon) using a DRIE (Deep Reactive Ion Etch) process. More specifically, the invention relates to a method of reducing undesired lateral etching while using such a manufacturing process.
Texas Instruments presently manufactures a two-axis MEMS type analog mirror device fabricated out of a single piece or sheet of material (such as a silicon wafer) typically having a thickness of about 115 xcexcm. The die layout defines an oval mirror, normally 3.8 mmxc3x973.2 mm supported on a gimbal frame by two torsional hinges which typically have a width on the order of about 8 microns. The gimbal frame is attached to the die frame by another orthogonal set of torsional hinges. The mirror die (i.e. each individual device) is fabricated by Deep Reactive Ion Etching (DRIE) the 115 xcexcm thick silicon wafer in a specialized ICP (Inductively Coupled Plasma) reactor.
As will be appreciated, the operation of the gimbal mirror depends upon the effectiveness, integrity and resilient support of the very narrow integrally formed torsional hinges. Therefore, it will also be appreciated that etching variations while forming the hinge can result in an unsatisfactory or even a failed mirror device.
One method of manufacturing the MEMS type mirror device discussed above is to bond a silicon wafer, which is typically less than 300 microns in thickness (and typically on the order of about 115 microns), to a thicker support or carrier wafer by using a coating or layer of a bonding agent or adhesive. Typically, the bonding agent provides a temporary bond such as is provided by a photo-resist or other organic layer of material such as xe2x80x9cWafer Gripxe2x80x9d available from Dynatex of Santa Rosa, Calif. The MEMS components are then formed by etching completely xe2x80x9cthrough the waferxe2x80x9d to the temporary bonding layer.
When the etching has progressed through or substantially through the is layer or wafer, the positively charged ion particles used to etch the wafer encounter the temporary bonding layer (such as photo-resist). This insulating layer of photo-resist is then positively charged by the impinging charged ions such that subsequent charged etching ions are repelled from the silicon wafer. Many of these repelled ions then encounter and laterally etch or erode the bottom of the wafer adjacent the location of the side wall of the etched area of line. Since as mentioned above, the torsional hinges of the mirror are on the order of only eight microns wide, this lateral etching can erode the silicon completely across the bottom of the hinge. Therefore, a method of etching through a wafer without lateral etching at the bottom surface of the wafer would improve the process.
The present invention relates to individual mirror devices formed on a wafer using processing steps which eliminate or substantially reduce undesirable lateral etching. Some of the processing steps are similar to steps used in manufacturing IC""s and other semiconductor devices.
The present invention provides a process for manufacturing one or more MEMS devices on a first layer of material, such as for example, a thin wafer of silicon typically having a thickness of less than about 300 xcexcm (microns) and according to one embodiment about 115 xcexcm. The process comprises providing a bonding agent such as a photo-resist on a backing layer or carrier wafer made of a conductive or semiconductive material. The bonding layer is applied and/or patterned on the surface of the backing layer of material so as to define uncoated areas of the backing layer. A silicon wafer or first layer of material is then attached to the backing layer with the bonding agent. A mask, such as a photo-lithography mask, is then applied or provided on the silicon wafer or first layer of material so as to define a pattern of etch areas and/or lines. Selected ones of the etch areas or lines of the mask are aligned with selected ones of the uncoated areas of the backing layer. The first layer of material, such as a silicon wafer, is then etched through the mask to form the MEMS device type mirrors. The devices are then separated from the backing layer.
Although as described above the first layer may be a silicon wafer, it should also be noted that the first layer used to manufacture the mirrors or the devices could be another suitable material. Further, the first layer or wafer may also undergo other processes before the device is etched. For example, electronics, sensors or other mechanical features can be created by standard IC or MEMS fabrication steps before proceeding to the etching step that forms the mirror.